In a CCD type solid-state imaging device or a CMOS type solid-state imaging device mounted in a digital camera, a large number of photoelectric conversion devices (photodiodes) serving as photo acceptance portions and signal readout circuits for reading out photoelectric conversion signals obtained by the photoelectric conversion devices to the outside are formed on a surface of a semiconductor substrate. In the CCD type solid-state imaging device, each of the signal readout circuits includes a charge transfer circuit, and a transfer electrode. In the CMOS type solid-state imaging device, each of the signal readout circuits includes an MOS circuit, and a signal wiring.
Accordingly, in the solid-state imaging device according to the related art, both the large number of photo acceptance portions and the signal readout circuits have to be formed together on the surface of the semiconductor substrate. There is a problem that the total area of the photo acceptance portions cannot be enlarged.
In addition, in a single plate type solid-state imaging device according to the related art, one of color filters, for example, of red (R), green (G) and blue (B) is stacked on each photo acceptance portion so that each photo acceptance portion can detect an optical signal with corresponding one of the colors. For this reason, for example, a blue optical signal and a green optical signal in a position of a photo acceptance portion for detecting red light are obtained by applying an interpolation operation on detection signals of surrounding photo acceptance portions for detecting blue light and green light. This causes false colors to thereby result in lowering of resolution. In addition, blue and green light beams incident on a photo acceptance portion covered with a red color filter are absorbed as heat to the color filter without giving any contribution to photoelectric conversion. For this reason, there is also another problem that light utilization efficiency deteriorates and sensitivity is lowered.
While the solid-state imaging device according to the related art has various problems as described above, development on increase in the number of pixels has advanced. At present, a large number of photo acceptance portions (e.g. equivalent to several million pixels) are integrated on one chip of a semiconductor substrate, so that the size of an aperture of each photo acceptance portion approaches the wavelength of light. Accordingly, it is difficult to expect a CCD type or CMOS type image sensor to have better image quality or sensitivity than ever to thereby solve the abovementioned problems.
Under such circumstances, the structure of a solid-state imaging device, for example, described in JP-A-58-103165 has been reviewed. The solid-state imaging device has a structure in which a photosensitive layer for detecting red light, a photosensitive layer for detecting green light and a photosensitive layer for detecting blue light are stacked on a semiconductor substrate having signal readout circuits formed in its surface, by a film-forming technique and in which these photosensitive layers are provided as photo acceptance portions so that photoelectric conversion signals obtained by the photosensitive layers can be taken out to the outside by the signal readout circuits. That is, the solid-state imaging device has a photoelectric conversion film-stacked type structure.
According to the structure, limitation on design of the signal readout circuits can be reduced greatly because it is unnecessary to provide any photo acceptance portion on the surface of the semiconductor substrate. Moreover, sensitivity can be improved because efficiency in utilization of incident light is improved. In addition, resolution can be improved because light with the three primary colors of red, green and blue can be detected from one pixel. The problem of false colors can be eliminated. The problems inherent to the CCD type or CMOS type solid-state imaging device according to the related art can be solved.
Therefore, photoelectric conversion film-stacked type solid-state imaging devices described in JP-A-2002-83946, JP-T-2002-502120, JP-T-2003-502847 and JP-B-3405099 have been proposed in recent years. An organic semiconductor or nano particles may be used as the material of each photosensitive layer.
In the solid-state imaging device in which the photoelectric conversion films are stacked on the semiconductor substrate and in which signals are read out by the transistor circuits formed on the semiconductor substrate, capacitance between each transistor circuit and a corresponding photoelectric conversion film provided as a layer above the transistor circuit becomes high because the area of pixels is widened and the length of wiring for connecting the transistor circuit to the photoelectric conversion circuit is elongated. For this reason, there is a problem that a signal voltage component becomes low because photoelectric charges generated by the photoelectric conversion films are retained in the middle without flowing smoothly to gates of the output transistors.